We are the named patentees in U.S. Pat. No. 3,628,816 which describes a process of diffusion alloying the surface of a ferrous part in a molten lead medium. In this process, chromium is dissolved into molten lead which is placed in contact with the ferrous part to be surfaced. Chromium is alloyed into the surface of the part by metallic diffusion. Our earlier work with chromizing of steel involved use of a relatively high chromium addition to insure a high chromizing potential. Solubility of chromium in molten lead is reported (Constitution of Binary Alloys--Hansen & Anderko--McGraw Hill, 1956) to be 0.05% by weight in lead at about 2,000.degree. F., a typical processing temperature. The lead bath contained at least 2% elemental chromium or 2% ferrochromium (approximately 70% chromium, 30% iron) in all of the examples in our prior patent; in many of the examples the quantity of chromium exceeded 2%.
The process of surface alloying the ferrous part may be carried out in a sealed reactor containing the molten lead and the chromium. Because of the volatilization of lead at temperatures near 2,000.degree. F., it is desirable to cool the reactor to a lower temperature, such as 800.degree. F., but above the melting point of lead, before opening the reactor. During this cooling, the solubility of chromium in lead is reduced and undesirable precipitates form and deposit on the ferrous part being surfaced. The result of using these high chromium concentrations is undesirable surface roughness and porosity.
While there has been mention of substantial reduction of the percentage of chromium in the molten bath (for example, U.S. Pat., Nos. 3,184,331 and 3,467,545 and British Pat. No. 878,028), lead has not been the transfer vehicle in these processses. Instead, the processes disclosed in these patents employ totally different and unrelated baths, such as calcium, lithium and copper. Furthermore, these patents teach away from the use of chromium in the bath of sufficiently low percentage to arrive at the high quality surface mentioned above. For example, using calcium as a transfer bath in the process described in U.S. Pat. No. 3,184,331, it is therein stated that "while the content of transfer agent in the bath may vary between wide limits, a practical lower limit for most coating operations within the invention will be about 10% by weight". Likewise, in British Pat. No. 878.028, involving transfer of chromium in a molten copper bath, it is stated that 2-10% dissolved chromium is preferable.
A further undesirable aspect of maintaining high chromium content in the bath is the substantial cost. A high concentration in the bath results in a high concentration in the coating which reduces the ductility and malleability of the ferrous surface coated part.
It is therefore an important object of the present invention to provide a process for diffusing chromium into the surface of a ferrous part in a molten lead bath, in which the chromium content of the lead bath is reduced to a value that causes the resultant coating to be smoother and less porous.
Another object of the present invention is to provide an improved ferrous part made by this process.
Another object is to reduce the chromium content in the coating applied on a ferrous part by surface diffusion in molten lead bath to reduce cost of the process and to improve the ductility and malleability of surface diffused ferrous parts made by such process.
In summary, there is provided a process of diffusion coating a ferrous based substrate comprising: contacting the substrate with a molten lead-based bath containing lead and chromium as a surface diffusing element, the bath containing less than 0.85% chromium by bath weight, the lead-based bath having the essential physical and chemical properties of lead, and diffusing the chromium into the substrate.
A surface diffused part produced by such process comprises a ferrous based substrate and a zone at the surface of said substrate containing 5 to 45% chromium by weight.
The invention consists of certain novel features and a combination of steps and parts hereinafter fully described, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the invention.